1. Field of the Invention
The present invention relates generally to a switching circuit device comprising a multi-gate field effect transistor (FET) and a semiconductor device for configuring the switching circuit device.
2. Description of the Background Art
For example, a GaAs switching circuit device capable of a high-speed switching operation is used for a transmitter-receiver in a microwave communication system. FIG. 17 is a diagram showing an example of a transmitter-receiver using a switching circuit device comprising a conventional MESFET (Metal-Semiconductor Field Effect Transistor; hereinafter abbreviated as FET).
The transmitter-receiver shown in FIG. 17 comprises a transmit-receive antenna 100, a transmitting circuit 110, and a receiving circuit 120. The transmit-receive antenna 100 and the transmitting circuit 110 are connected to each other through an switching circuit device 130 comprising a FET, and the transmit-receive antenna 100 and the receiving circuit 120 are connected to each other through a switching circuit device 140 comprising an FET. Control voltages V1 and V2 are respectively applied to gate electrodes G1 and G2 in the FETs constituting the switching circuit devices 130 and 140.
In such a transmitter-receiver, when the control voltage V1 is 0 volt, and the control voltage V2 is -10 volts, the switching circuit device 130 is turned on, and the switching circuit device 140 is turned off. Consequently, a transmission signal from the transmitting circuit 110 is transmitted to the antenna 100. On the other hand, when the control voltage V1 is -10 volts, and the control voltage V2 is 0 volts, the switching circuit device 130 is turned off, and the switching circuit device 140 is turned on. Consequently, a receiving signal received by the antenna 100 is transmitted to the receiving circuit 120.
In order to decrease the size and increase the performance of communication equipment in microwave communication, a switching circuit device capable of performing a low-voltage operation and providing high-output power transmission is required. The switching circuit device 130 in the above-mentioned transmitter-receiver is configured by connecting a plurality of FETs 131 to 134 in series, as shown in FIG. 18, so that large power can be turned on or off at a low control voltage V1. Similarly, the switching circuit device 140 is also configured by connecting a plurality of FETs 141 to 144 in series, so that a large power can be turned on or off at a low control voltage V2. As a result, the switching circuit devices 130 and 140 can perform a low-voltage operation and make high-output power. transmission. In FIG. 18, an interstage resistor RH is a resistor for keeping an interstage potential between the FETs constant.
When the number of stages of the FETs constituting the switching circuit device as described above is increased, however, an area occupied by the FETs is increased. Therefore, the size of the switching circuit device is increased.
In order to solve the above-mentioned disadvantage, a switching circuit device using a multi-gate FET having a plurality of gate electrodes between a drain electrode and a source electrode (between a set of ohmic electrodes) has been proposed in JP-A-9-238059.
In the switching circuit device described in the above-mentioned application, however, the chip size can be decreased, while an interstage potential between the gate electrodes cannot be kept constant. Therefore, the interstage potential between the gate electrodes becomes unstable. As a result, a portion between the drain electrode and the source electrode cannot be brought into a completely off state when the switching circuit device is turned off.